For consumer secondary cells employed in mobile terminal devices such as personal computers, mobile phones and the like, and video cameras, there have been intensively developed lithium ion cells that are capable of being super-thinned and miniaturized although high in energy.
As a packaging material for lithium ion cells (which may be sometimes referred to simply as “packaging material”), multi-layered laminate films have been in use in place of existing metal cans because of their advantages of lightweight and freedom in selection of a cell shape. The packaging material using such a laminate film not only has the freedom in selection of a cell shape, but also is light in weight, high in heat radiation performance and low in cost. Therefore, attempts have been made to its application to batteries of hybrid vehicles and electric vehicles that have been pronouncedly developed recently and have a low environmental burden.
As to the structure of the laminate film, the usual practice is such that a sealant layer (thermally fusible film) is stacked on one surface of a metal foil layer, such as an aluminum foil, through an adhesive layer, and a substrate layer (plastic film) is stacked on the other surface through an adhesive layer (i.e. substrate layer/adhesive layer/metal foil layer/adhesive layer/sealant layer).
The lithium ion cells making use of a packaging material of the laminate film type can be formed, for example, in the following way: Initially, the laminate film is deep drawn by the use of cold forming (deep drawing) to obtain a formed article. Next, the formed article is accommodated therein with an electrolytic solution or an electrolyte layer composed of a polymer gel impregnated with the electrolytic solution along with a cell body that is made of a positive electrode material, a negative electrode material, and a separator. In the state where such members as mentioned above are accommodated in the formed article, the formed article is thermally sealed by heat sealing to form a cell.
The electrolytic solution used is one wherein a lithium salt is dissolved in an aprotic solvent (e.g. propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or the like).
The electrolytic solution has high permeability in the sealant layer. Accordingly, it has been experienced that an electrolytic solution infiltrated in the sealant layer of lithium ion cells causes the lamination strength between the metal foil layer and the sealant layer to be lowered, finally resulting in the leakage of the electrolytic solution. The lithium salt, such as LiPF6, LiBF4 or the like, serving as an electrolyte may sometimes generate hydrofluoric acid by hydrolytic reaction. Hydrofluoric acid causes the corrosion of a metal surface and brings about the lowering of the lamination strength between the respective adjacent layers of the laminate film. In this sense, the packaging material should have an anti-corrosion performance against an electrolytic solution or hydrofluoric acid.
As a packaging material satisfying the above requirement, there has been disclosed, for example, in PTL 1 a packaging material that is able to suppress the lamination strength between a sealant layer and a metal foil layer from being lowered with time due to an electrolytic solution. In this packaging material, the sealant layer and the metal foil layer are bonded through a layer made of an adhesive containing a carboxy group-containing polyolefin resin and a polyfunctional isocyanate compound.